Structural studies of the inclusion complexes of the (+)- and (−)-borneol enantiomers in α- and β-cyclodextrin

Abstract

The crystal structures of the inclusion compounds of the (+)- and (−)-borneol enantiomers in α- and β-cyclodextrin (CD) were determined by X-ray crystallography at about 1 A resolution. In all the cases, the α- or β-CDs form head-to-head dimers arranged in a “chessboard” crystal packing mode. In the cases of the (+)- and (−)-borneol/αCD inclusion complexes, one guest molecule is accommodated inside the formed dimeric cavity (2:1 host:guest stoichiometry) disordered over two and three sites respectively. Ιn the cases of the (+)- and (−)-borneol/βCD inclusion complexes, a highly disordered guest molecule is located inside the dimeric cavity and two additional guest molecules lay at the rims of the primary hydroxyls of the dimer (2:3 host:guest stoichiometry) participating in the crystal contacts by forming H-bonds with external water molecules and –OH groups of the β-CDs of the adjacent dimers. Regarding the ability of α- and β-CD for borneol enantioseparation, the crystallographic analysis shows that no significant differences concerning the inclusion geometry and crystal packing are observed between the inclusion complexes of the borneol enantiomers with the same host CD.